Feature/simple gan for api (#6149)
Feature/simple gan for api (#6149)

* Expose sigmoid_cross_entropy_with_logits

Also, change the `labels` to `label` for api consistency

* Very simple GAN based on pure FC layers

luotao1/Paddle,pkuyym/Paddle,luotao1/Paddle,chengduoZH/Paddle,putcn/Paddle,pkuyym/Paddle,reyoung/Paddle,Canpio/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,tensor-tang/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,QiJune/Paddle,jacquesqiao/Paddle,baidu/Paddle,QiJune/Paddle,Canpio/Paddle,tensor-tang/Paddle,tensor-tang/Paddle,baidu/Paddle,putcn/Paddle,chengduoZH/Paddle,luotao1/Paddle,Canpio/Paddle,reyoung/Paddle,reyoung/Paddle,tensor-tang/Paddle,QiJune/Paddle,lcy-seso/Paddle,pkuyym/Paddle,QiJune/Paddle,chengduoZH/Paddle,jacquesqiao/Paddle,chengduoZH/Paddle,putcn/Paddle,luotao1/Paddle,Canpio/Paddle,luotao1/Paddle,lcy-seso/Paddle,reyoung/Paddle,Canpio/Paddle,luotao1/Paddle,Canpio/Paddle,tensor-tang/Paddle,PaddlePaddle/Paddle,reyoung/Paddle,jacquesqiao/Paddle,Canpio/Paddle,putcn/Paddle,jacquesqiao/Paddle,chengduoZH/Paddle,lcy-seso/Paddle,luotao1/Paddle,putcn/Paddle,reyoung/Paddle,pkuyym/Paddle,lcy-seso/Paddle,PaddlePaddle/Paddle,pkuyym/Paddle,pkuyym/Paddle,putcn/Paddle,Canpio/Paddle,QiJune/Paddle,baidu/Paddle,lcy-seso/Paddle,lcy-seso/Paddle,jacquesqiao/Paddle,baidu/Paddle,PaddlePaddle/Paddle,jacquesqiao/Paddle,baidu/Paddle,QiJune/Paddle